Ecosystem Services 7 (2014) 106–115
Contents lists available at ScienceDirect
Ecosystem Services
journal homepage: www.elsevier.com/locate/ecoser
A synoptic survey of ecosystem services from headwater catchments
in the United States
Brian H. Hill a,n, Randall K. Kolka b, Frank H. McCormick c, Matthew A. Starry d,1
a
US Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Laboratory, Mid-Continent Ecology
Division, Duluth, MN 55804, USA
b
US Forest Service, Northern Research Station, Center for Research on Ecosystem Change, Grand Rapids, MN 55744, USA
c
US Forest Service, Rocky Mountain Research Station, Air, Water and Aquatic Environments Science Program, Boise, ID 83702, USA
d
SRA International, Fairfax, VA 22033, USA
art ic l e i nf o
a b s t r a c t
Article history:
Received 21 May 2013
Received in revised form
25 November 2013
Accepted 4 December 2013
Available online 27 December 2013
Ecosystem production functions for water supply, climate regulation, and water purification were estimated
for 568 headwater streams and their catchments. Results are reported for nine USA ecoregions. Headwater
streams represented 74–80% of total catchment stream length. Water supply per unit catchment area was
highest in the Northern Appalachian Mountains ecoregion and lowest in the Northern Plains. C, N, and P
sequestered in trees were highest in Northern and Southern Appalachian and Western Mountain catchments,
but C, N, and P sequestered in soils were highest in the Upper Midwest ecoregion. Catchment denitrification
was highest in the Western Mountains. In-stream denitrification was highest in the Temperate Plains.
Ecological production functions paired with published economic values for theses services revealed the
importance of mountain catchments for water supply, climate regulation, and water purification per unit
catchment area. The larger catchment sizes of the plains ecoregions resulted in their higher economic value
compared to the other ecoregions. The combined potential economic value across headwater catchments
was INT $14,000 ha 1 yr 1, or INT $30 million yr 1 per catchment. The economic importance of headwater
catchments is even greater considering that our study catchments statistically represent more than 2 million
headwater catchments in the continental United States.
Published by Elsevier B.V.
Keywords:
C sequestration
Climate regulation
Denitrification
Headwater catchments
Water purification
Water supply
1. Introduction
Headwater streams and their catchments have received much
attention in recent years, with issues ranging from their contribution to, and connection with, larger downstream ecosystems
(Nadeau and Rains, 2007), to the loss of headwater streams to
burial and routing through underground pipes (Roy et al., 2009;
Kaushal and Belt, 2012). An emerging concern is the underestimation of the extent of headwater stream channels, even when
mapped at scales finer than 1:100,000 scale (Roy et al., 2009).
Headwater streams are defined as the terminal branchings of a
stream drainage network, the point where water flowing in a
catchment first coalesces into defined stream channels (Gomi
et al., 2002). Nadeau and Rains (2007) extend the definition of
headwater streams to first- and second-order streams (Strahler,
1957) on 1:100,000 scale maps, even though researchers have
n
Correspondence to: USEPA/NHEERL/MED 6201 Congdon Blvd. Duluth, MN
55804 (USA). Tel.: þ 1 218 529 5224; fax: þ 1 218 529 5003.
E-mail address: hill.brian@epa.gov (B.H. Hill).
1
Current address: Superior Water, Power and Light, Superior, WI 54880, USA.
2212-0416/$ - see front matter Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.ecoser.2013.12.004
demonstrated the underestimation of headwater streams at this
larger (coarser) map scale (Meyer and Wallace, 2001; Roy et al.,
2009). Because of stream network scaling properties (Dodds and
Rothman, 2004), the proportion of total basin-wide headwater
stream length is approximately 70% of total stream length on both
1:100,000 and 1:24,000 scale maps (Leopold et al., 1964; Nadeau
and Rains, 2007; Lassaletta et al., 2010).
In addition to their dominance in terms of numbers and
cumulative length, headwater streams also exert controls on
stream runoff and downstream fluxes of dissolved and particulate
matter organic matter and nutrients (Alexander et al., 2007; Dodds
and Oakes, 2008; Lassaletta et al., 2010). Using spatial regression
models, Alexander et al. (2007) estimated that headwater streams
deliver 60% of the runoff and 45% of the nitrogen load in downstream reaches in northeastern US streams and rivers. They
attribute this result to the high density of headwater streams
and the frequency of their connections to higher-order stream
channels. In a review of the influence of headwater streams on
downstream reaches, MacDonald and Coe (2007) reported an even
greater proportion of runoff and nutrient loading is directly
attributable to headwater streams. Similarly, Dodds and Oakes
(2008) reported that nutrient chemistry in fourth-order Kansas
streams was best predicted by riparian land cover adjacent to
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
upstream first-order streams. These results are similar to those
reported for European streams (Lassaletta et al., 2010).
This downstream influence by headwater streams indicates a
hydrologic connectivity that links headwater catchments, their
soil, and groundwater resources, with larger-order streams (Gomi
et al., 2002; Wipfli et al., 2007; Freeman et al., 2007). Headwater
streams are not specifically protected by the Clean Water Act
(CWA), but until recently they were included as necessary for the
maintenance of healthy, productive, and navigable streams and
rivers. Their protection under the CWA has recently been limited
by the Supreme Court (Rapanos v. United States 547 US 715, 2006)
to only those headwater streams that are directly connected to, or
have demonstrated a significant influence on, navigable waters
(Nadeau and Rains, 2007). Even with CWA protections, headwater
streams have been lost from the landscape, primarily by humandriven changes in catchment land use including agriculture,
urbanization, and mining (Meyer and Wallace, 2001; Roy et al.,
2009; Kaushal and Belt, 2012). An analysis of 106 catchments from
around the world revealed that nearly one-third of them experienced extensive conversions (450% of the catchment) of forests
to agriculture or urban development (Postel and Thompson, 2005).
Ecosystem services are the result of direct and indirect contributions of ecosystems to human well-being (Burkhard et al.,
2012). The Millennium Ecosystem Assessment (2003) classified
ecosystem services into four categories: provisioning services
which provide goods for direct human use (food, freshwater,
timber); regulating services to maintain biophysical properties
for living beings (climate stability, water purification); cultural
services including aesthetic and spiritual benefits; and supporting
services which are necessary for the maintenance of functioning
ecosystems (nutrient cycling, primary production, soil formation).
Catchments represent a discrete unit for accounting for the
delivery of ecosystem goods and services to society (Postel and
Thompson, 2005). Catchment ecosystem services are many,
including biodiversity, climate regulation, recreation, timber and
crop production, and water supply and purification. Timber
markets are globally well established, carbon exchange markets
are developing (Intercontinental Exchange, 2012), and water
supply is easily valued as a commodity (Krieger, 2001; Postel
and Thompson, 2005; Nunez et al., 2006; de Groot et al., 2012;
Townsend et al., 2012); but the value of water purification via
nitrogen and phosphorus sequestration in biomass and soils, and
through denitrification, are only beginning to receive economic
consideration (Dodds et al., 2009; Turpie et al., 2010, Compton
et al., 2011). Some of these catchment ecosystem services co-vary
while others compete, and understanding the interplay and
relationships among ecosystem services under varied management of these resources is critical to the sustainable delivery of
catchment ecosystem goods and services (Bennett et al., 2009; de
Groot et al., 2010; Deal et al., 2012; Townsend et al., 2012).
Our objectives in this paper are to highlight the importance of
headwater catchments by focusing on the quantity and value of a few
ecosystem services derived from them, and to extrapolate that
importance from regional to national scales within the continental
United States. We focus on headwaters because that is a particular
category of streams that is of interest in the US regulatory community.
As an under-protected resource, we wanted to highlight their particular value. We combine data collected from headwater streams as a
part of the US Environmental Protection Agency's (USEPA) National
Rivers and Streams Assessment (NRSA) with catchment attributes
related to water supply, the sequestration of C, N, and P, and the
removal of N via denitrification. We use these data to develop
ecological production functions related to the delivery of ecosystem
services from headwater catchments, and combine these services with
published valuations to estimate potential cumulative benefits derived
from headwater catchments in the United States.
107
2. Materials and methods
2.1. Study sites
Catchments included in this study were those drained by the
568 first- and second-order (Strahler, 1957) streams that were
sampled during the NRSA (Fig. 1). The sampling design was
spatially-balanced and employed an unequal probability survey
with the unequal selection based on stream order. The design
selected a single point along the center line of each stream as
depicted by the National Hydrography Dataset (NHDPlus, Version
1; http://horizon-systems.com/nhdplus; based on 1:100,000 scale
maps). All sample sites were selected using NHDPlus as the sample
frame. Each site included in the survey represented a known
stream or river length based on the population of streams included
in the survey design, the probability of that site being selected for
sampling, and the number of sites actually sampled. These stream
and river lengths were summed to estimate the cumulative extent
of streams sampled (Olsen and Peck, 2008).
The NRSA design allows the assessment of ecological conditions of streams at three hierarchical spatial scales: national,
regional, and ecoregional (Olsen and Peck, 2008). Here we report
results nationally and for nine ecoregions: Northern Appalachian
Mountains, Southern Appalachian Mountains, Coastal Plains,
Northern Plains, Southern Plains, Temperate Plains, Upper Midwest, Western Mountains, and Xeric ecoregions (Fig. 1).
2.2. Catchment attributes
Total catchment area (A, ha) for each site was calculated by
summing the areas of all NHDPlus catchments intersected while
navigating upstream from each sampling site. Cumulative catchment area (Cum A, ha) within an ecoregion was calculated as the
product of mean A and the total number of catchments (n) in that
ecoregion (Table 1; Fig. 2). Percent of the catchment in forests
(% forest), grasslands (% grassland), row crops (% agriculture), and
wetlands (% wetland) were extracted from the National Land Cover
Database (NLCD, USGS, 2006; Fig. 2). The NLCD, derived from
multi-temporal and terrain-corrected satellite imagery, provides
consistent land cover estimates for the United States. Targeted
assessments found accuracy of land cover estimates ranged from
78 to 89% (Xian et al., 2009).
Catchment stream lengths (L, km) were estimated using
NHDPlus flow line and stream order data layers (Fig. 2). NHDPlus
codes flow lines as connectors, canals and ditches, underground
pipes, intermittent and perennial streams, artificial paths, and
coastlines. We excluded underground pipes and coastlines from
our analyses, and the remaining types of water conveyances are
collectively treated as streams. Each stream segment of a given
order was included in the estimate of L by stream order, and
cumulative catchment stream length (Cum L, km) was calculated
as the product of L and n.
Catchment-scale estimates of soil organic carbon (SOC) and % sand
were derived from US Department of Agriculture soil survey data
(SSURGO and STATSGO2; http://soildatamart.nrcs.usda.gov/; Fig. 2)
and associated with each headwater catchment as the mean of the
30-m pixels included in each catchment. Soil drainage index (DI),
previously called the natural soil wetness index, is a measure of the
long-term wetness of a soil (Schaetzl et al., 2009). Catchment-scale
estimates of DI were estimated from area-weighted STATSGO2 map
units (http://www.drainageindex.msu.edu; Fig. 2) that intersected our
study catchments.
Data on the wet deposition of atmospheric N were available
from the National Atmospheric Deposition Program (NADP, http://
nadp.sws.uiuc.edu). We used annual (2005–2009) precipitationweighted mean TN concentrations in precipitation. Estimates of
108
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
Fig. 1. Locations of headwater streams sampled during the National Rivers and Streams Assessment. The nine ecoregions for NRSA reporting are delineated.
wet TN deposition on each of our study catchments were based on
NADP station data that were intersected with spatiallyinterpolated national grids (http://nadp.sws.uiuc.edu/isopleths).
These interpolated data were averaged across the years of our
study.
Soil N and P content were based on average soil C, N, and P
stoichiometry (Cleveland and Liptzin, 2007). Annual increments of
soil C, N, and P sequestration were based on a 500 yr average age
for soil organic matter, and assuming a linear increase in C, N, and
P sequestration through time (Trumbore, 2000; Six and Jastrow,
2007).
2.3. Carbon, nitrogen and phosphorus sequestration
2.4. Catchment and in-stream denitrification
Carbon sequestered in woody vegetation was estimated by
multiplying catchment area by the proportion of the catchment
covered by forests, and then by mean above and below ground C
standing stocks for living tree biomass in the Eastern Highlands
(Northern and Southern Appalachian Mountains ecoregions;
18,400 Mg km 2), Plains and Lowlands (Coastal, Northern, Southern and Temperate Plains, Upper Midwest, and Xeric ecoregions:
20, 700 Mg km 2), or Western Mountains (18,900 Mg km 2; US
Department of Agriculture, 2008; Heath et al., 2011; US
Environmental Protection Agency (USEPA), 2011; Fig. 2). Nitrogen
and phosphorus sequestered in tree biomass were estimated from
global measures of tree C, N, and P stoichiometry (Schade et al.,
2005). These C, N, and P standing stock estimates were divided by
median forest stand age (Smith et al., 2009) to estimate annual C,
N, and P sequestration by trees in US headwater catchments. It
was assumed that non-woody catchment vegetation contributed
negligibly to overall catchment C, N, and P sequestration. Sequestration of C, N, and P in agricultural crops was estimated by
multiplying catchment area by the proportion of the catchment
covered by row crop agriculture, and then by mean C, N, and P
standing stocks (Vitousek et al., 2009).
Soil C content was estimated from US Department of Agriculture soil survey data (SSURGO and STATSGO2; http://soildatamart.
nrcs.usda.gov/; Fig. 2) applied at the catchment scale (Bliss, 2003).
Catchment-scale denitrification (Catch DN) was estimated from
a regression model employing average % sand and DI for the 30-m
pixels of each catchment (Groffman et al., 1992; Table 1; Fig. 2).
Cumulative catchment DN was estimated as the product of Catch
DN and Cum A. Mulholland et al. (2008) developed a model for
predicting in-stream denitrification based on stream NO3 concentrations (Table 1; Fig. 2). We applied their DN model to
measured NO3 concentrations from the NRSA streams to predict
in-stream DN (Stream DN). Catchment-wide Stream DN was
estimated as the product of Stream DN and Cum L.
2.5. Hydrology and water supply
Mean annual precipitation (PPT, mm yr 1) was acquired as an
NHDPlus data layer for each of our study catchments. NHDPlus
uses a spatial interpolation model based on 30 yr of precipitation
data (1981–2010) from the National Weather Service precipitation
gauge network (http://water.weather.gov/ahps/). Mean annual
discharge (Q, m3 s 1) from each catchment was also available as
an NHDPlus data layer. NHDPlus uses a spatial interpolation model
of unit runoff based on 30 yr of US Geological Survey stream gage
data (http://waterdata.usgs.gov/nwis). Streamflow data were converted from volumetric rates (m3 s 1) to the more commonly
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
109
Table 1
Definitions of the physical and chemical dimensions of the study catchments and ecosystem services metrics.
Label
Name, units
n
A
Cum A
L
Cum L
% HW
% Forest
% Grassland
% Wetland
% Agriculture
PPT
RO
ET
ET index
RO ratio
% Sand
DI
N deposition
Tree C
Number of study catchments
Catchment area (ha)
Cumulative A (ha)
Catchment stream length (km)
Cumulative L (km)
% of total streams that are headwaters
% of A in forests
% of A in grasslands
% of A in wetlands
% of A in row crops
Precipitation (mm yr 1)
Runoff, (mm yr 1)
Evapotranspiration, (mm yr 1)
Evaporative index
Runoff ratio
% of soil column as sand
Drainage index
Total N deposition (kg N ha 1 y 1
C stocks in trees (Mg ha 1)
Northern and Southern
Appalachian Mountains:
Western Mountains:
Other ecoregions:
N stocks in trees, Mg ha 1
P stocks in trees, Mg ha 1
C stocks in crops, Mg ha 1
N stocks in crops, Mg ha 1
P stocks in crops, Mg ha 1
C stocks in soil, Mg ha 1
N stocks in soil, Mg ha 1
P stocks in soil, Mg ha 1
Catchment denitrification, Mg ha 1
In-stream denitrification, Mg km 1
Tree N
Tree P
Crop C
Crop N
Crop P
Soil C
Soil N
Soil P
Catch DN
Stream DN
Derivation
Reference/source
a
NHDPlus
a
NHDPlus
An
Ln
a
NHDPlus
NLCD
b
NLCD
b
NLCD
b
NLCD
a
NHDPlus
a
NHDPlus
b
Converted from discharge, m3 s 1
PPT-RO
ET/PPT
RO/PPT
18,400 % Forest
18,900 % Forest
20,700 % Forest
3000:45:1C:N:P stoichiometry
3000:45:1C:N:P stoichiometry
200 % Ag
14.5 % Ag
2.3 % Ag
Soil OC, total profile
186:13:1C:N:P stoichiometry
186:13:1C:N:P stoichiometry
0.34 (DI) 0.40 (% sand) þ 11.8
( 0.493 ln NO3) 2.975) NO3
Jones et al. (2012)
Jones et al. (2012)
c
SSURGO/STATSGO
Schaetzl et al. (2009)
d
NADP
e
FIA
FIA
e
FIA
Schade et al. (2005)
Schade et al. (2005)
Vitousek et al. (2009)
Vitousek et al. (2009)
Vitousek et al. (2009)
c
SSURGO/STATSGO
Cleveland and Liptzin (2007)
Cleveland and Liptzin (2007)
Groffman et al. (1992)
Mulholland et al. (2008)
e
a
National Hydrography Datasets (http://www.horizon-systems.com/NHDPlus/NHDPlusV2_data.php).
National Land Cover Database (http://www.mrlc.gov/nlcd06_data.php).
c
National and State Soil Survey Geographic Databases (http://soildatamart.nrcs.usda.gov/).
d
National Atmospheric Deposition Program (http://nadp.sws.uiuc.edu/isopleths).
e
Forest Inventory and Analysis (http://www.fia.fs.fed.us/tools-data/default.asp).
b
reported runoff (RO, mm yr 1) to facilitate the calculation of
actual evapotranspiration (ET ¼PPT-RO; mm yr 1), evaporative
index (ET Index ¼ET/PPT), and runoff ratio (RO Ratio ¼RO/PPT;
Jones et al., 2012; Table 1; Fig. 2).
2.6. Economic value of ecosystem services
As a relative comparison among ecosystem services and between
ecoregions, we estimated the potential economic value of ecosystem
services provided by headwater catchments as the product of units of
production (production function) and published economic values for
those units of production. Economic values are reported in International $, where 1INT $¼1 US $ (de Groot et al., 2012). We restrict our
comparisons to considerations three ecosystem services, (1) water
supply; (2) climate regulation (C sequestration), and (3) water
purification (N and P sequestration and denitrification). The economic
value of water supply was estimated as INT $0.35 m 3 (INT $40 acre ft 1; Sedell et al., 2000; Krieger, 2001; Nunez et al., 2006; Brauman
et al., 2007). The value of the C sequestered in catchment biomass and
soil was INT $0.12 Mg 1 (Mg¼1000 t; Intercontinental Exchange,
2012). The value of N sequestered in catchment biomass and soil,
and removal via DN was INT $160 Mg 1 (US Environmental Protection
Agency (USEPA), 2007; Dodds et al., 2009; Compton et al., 2011). P
sequestered in catchment biomass and soils were conservatively
valued at INT $1600 Mg 1 (Sano et al., 2005; US Environmental
Protection Agency (USEPA), 2007; Stanton et al., 2010).
2.7. Statistical analyses
We calculated mean values for catchment attributes; annual
precipitation and runoff; C, N, and P sequestration; and N removal;
and their economic values. Catchment means among ecoregions
were compared using a non-parametric one-way analysis of
variance on ranked scores (Wilcoxon) with the Kruskal–Wallis
test. All analyses were done using SAS for Windows, release 9.2
(SAS Institute, Inc., Cary, NC, USA).
3. Results
3.1. Catchment attributes and ecosystem services production
functions
The average size of the headwater catchments in our study
ranged from o2000 ha for catchments in the Eastern US (Northern and Southern Appalachian Mountains and Coastal Plains) to
48000 ha in the Northern and Southern Plains ecoregions
(Table 2). Headwater stream length was shorter in the Eastern
US and longer in the Northern and Southern Plains and Xeric
ecoregions. Overall, headwater streams represented 74–80% of
total catchment stream length (Table 2). Percent of the catchment
in forest land cover was greatest in the Northern and Southern
Appalachian and Western Mountains; grasslands were greatest in
the Northern and Southern Plains; wetlands were more prevalent
in the Coastal Plains and in the Upper Midwest; and the Temperate
110
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
Fig. 2. Conceptual model of a headwater stream catchment (gray-shaded area) annotated with sources for data inputs (solid arrows) and ecosystem service outputs (dashed
arrows). Details of data uses and ecosystem services calculations are presented in Table 1. Conceptual catchment model.
Table 2
Mean catchment area (A, ha) and stream length (L, km); proportion of total catchment L that is headwaters (% HW); proportion of the catchment cover by forests, grasslands,
wetlands, or row crop agriculture (%); annual precipitation (PPT, mm yr 1), evapotranspiration (ET, mm yr 1), runoff (RO, mm yr 1), evaporative index (ET index ¼ ET/PPT),
runoff ratio (RO ratio¼RO/PPT), proportion of the soil column that is sand (% sand), drainage index (DI) and atmospheric N deposition (kg N ha 1 yr 1) for headwater
catchments in the nine NRSA ecoregions. Results of comparisons among ecoregions were based on Wilcoxon Rank sum scores and the Kruskal–Wallis test (χ2 and P o χ2).
Variable
(n)
NAPa
(71)
SAPb
(1 3 5)
CPLc
(1 0 0)
NPLd
(12)
SPLe
(26)
TPLf
(77)
UMWg
(61)
WMTh
(83)
XERi
(35)
χ2
P o χ2
A
L
% HW
% Forest
% Grassland
% Wetland
% Agriculture
PPT
ET
RO
ET index
RO ratio
% sand
DI
N deposition
1263
5
78
67
1
4
15
1101
454
1124
0.41
0.99
47
49
13.5
1427
6
77
63
4
o1
21
1227
634
833
0.52
0.68
34
43
12.3
1615
6
78
42
3
13
23
1322
748
742
0.57
0.56
44
56
10.9
11,437
14
74
19
39
2
22
471
363
109
0.77
0.23
34
30
4.80
8260
18
78
16
46
1
17
712
618
117
0.87
0.17
37
34
8.60
3054
9
77
18
7
1
65
933
677
384
0.74
0.38
21
57
14.2
2093
7
75
40
2
18
29
804
465
450
0.58
0.56
51
50
11.6
2598
10
80
69
8
1
o1
1142
474
999
0.52
0.73
46
37
3.33
2466
11
80
42
5
o1
o1
526
341
250
0.65
0.47
39
26
3.10
91.2
48.7
5.32
202
128
271
255
350
170
234
158
157
116
223
405
o 0.0001
o 0.0001
0.7229
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
a
Northern Appalachian Mountains.
Southern Appalachian Mountains.
c
Coastal Plains.
d
Northern Plains.
e
Southern Plains.
f
Temperate Plains.
g
Upper Midwest.
h
Western Mountains.
i
Xeric ecoregions.
b
Plains were dominated by row crop agriculture (Table 2). Precipitation (PPT) was higher in the East (Northern and Southern
Appalachian Mountains, and Coastal Plains) and lower in the
Northern Plains and Xeric ecoregions; evapotranspiration
(ET) was greater in the Coastal, Southern and Temperate Plains
ecoregions, and lower in the Northern Plains and Xeric ecoregions
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
(Table 2).The combination of high PPT and low ET resulted in
higher runoff (RO) from the Appalachian and Western Mountain
catchments, while low PPT and high ET resulted in lower RO from
Northern and Southern Plains catchments. The resulting runoff
coefficients, the proportion of PPT that leaves the catchment via
runoff, ranged from 0.99 in the Northern Appalachian Mountains
to 0.17 in the Southern Plains (Table 2). Annual runoff (or water
supply, m3 ha 1 yr 1) was 2–5 times higher in the Northern and
Southern Appalachian Mountains, the Coastal Plains, and the
Western Mountains than in the other ecoregions (Table 4). The
sand content of catchment soil profiles ranged from 21% (Temperate Plains) to 51% (Upper Midwest), and the soil drainage index, a
measure of relative soil moisture retention, ranged from 26 in the
Xeric region to 450 in the Coastal and Temperate Plains and the
Upper Midwest ecoregions. Neither of these soil properties exhibited statistical ecoregional differences (Table 2).
111
Carbon stocks sequestered in trees were highest in the Northern and Southern Appalachian and Western Mountains ecoregions
and lowest in the Northern, Southern, and Temperate Plains
(Table 3). This pattern was repeated for tree N and P stocks,
because of the stoichiometric assumptions (Schade et al., 2005;
Table 3). C stock sequestered in catchment soils was highest in the
Upper Midwest ecoregion, but few other discernible patterns were
noted. This relationship among ecoregions was also applicable to
soil N and P, due to our stoichiometric assumptions (Cleveland and
Liptzin, 2007; Table 3). C, N, and P sequestered in harvested
agricultural crops were highest in the Temperate Plains and lowest
in the Western Mountains and Xeric ecoregions (Table 3).
C, N, and P stocks sequestered in trees were annualized by
dividing the cumulative catchment values by median stand age
(Smith et al., 2009) to yield an annual sequestration increment
(Mg yr 1). Similarly, soil C, N, and P stocks were annualized on a
Table 3
Mean amounts of C, N, and P stocks (kg ha 1) in forest and crop biomass and soil and removed through catchment and in-stream denitrification (DN, kg N ha 1 yr 1) for
headwater catchments in the nine NRSA ecoregions. Results of comparisons among ecoregions were based on Wilcoxon Rank sum scores and the Kruskal–Wallis test (χ2 and
P o χ2).
Variable
NAPa
SAPb
CPLc
NPLd
SPLe
TPLf
UMWg
WMTh
XERi
χ2
P o χ2
Forest C
Forest N
Forest P
Soil C
Soil N
Soil P
Crop C
Crop N
Crop P
Catch DN
Stream DN
123,485
1,851
41.2
123,534
8,639
664
308
22.4
3.54
11.8
2.01
115,617
1,733
38.5
50,720
3,547
273
419
30.4
4.82
13.1
6.77
86,565
1,298
28.9
117,163
8,193
630
465
33.7
5.34
14.9
4.10
39,061
586
13.0
75,334
5,268
405
439
31.8
5.05
10.7
3.62
33,751
506
11.2
87,988
6,153
473
347
25.2
3.99
13.3
8.53
37,390
561
12.5
138,348
9,675
744
1297
94.0
14.9
22.7
9.08
82,503
1,237
27.5
231,249
16,171
1,243
586
42.5
6.74
12.4
3.40
129,897
1,947
43.3
85,721
5,994
461
4.24
0.31
0.05
90.7
2.40
80,199
1,202
26.7
64,718
4,526
348
6.93
0.50
0.08
6.30
2.15
172
172
172
227
227
227
255
255
255
137
144
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
o 0.0001
a
Northern Appalachian Mountains.
Southern Appalachian Mountains.
c
Coastal Plains.
d
Northern Plains.
e
Southern Plains.
f
Temperate Plains.
g
Upper Midwest.
h
Western Mountains.
i
Xeric ecoregions.
b
Table 4
Stand age (yr) and mean annual ecosystems goods and services (water supply, m3 ha 1 yr 1; climate regulation, C-sequestered in trees a and soil b, kg ha 1 yr 1; and water
purification, N and P sequestered in trees a and soil b, kg ha 1 yr 1; N removal by catchment and in-stream denitrification, kg ha 1 yr 1) derived from headwater
catchments of the nine ecoregions. C, N, and P sequestration trees, crops, and soil was estimated as the product of sequestration (kg ha 1) and catchment area, divided by
stand or soil OM age. Stream denitrification was calculated as the product of denitrification and cumulative stream length.
Variable
NAPc
SAPd
CPLe
NPLf
SPLg
TPLh
UMWi
WMTj
XERk
Stand age
Water supply
Climate regulation
C sequestration
Water purification
N sequestration
P sequestration
Denitrification
70
11,244
30
8331
30
7424
50
1088
30
1171
50
3840
50
4502
90
9991
70
2502
2011
3955
3120
932
1301
1025
1641
1615
1275
a
43.7
1.92
13.7
64.9
1.83
20.2
59.6
2.22
19.7
22.2
1.07
14.2
Median stand age ranges from 30 to 90 yr (Smith et al., 2009).
Median soil organic matter age¼500 yr (Trumbore, 2000; Six and Jastrow, 2007).
c
Northern Appalachian Mountains.
d
Southern Appalachian Mountains.
e
Coastal Plains.
f
Northern Plains.
g
Southern Plains.
h
Temperate Plains.
i
Upper Midwest.
j
Western Mountains.
k
Xeric ecoregions.
b
29.2
1.32
24.4
30.6
1.74
31.8
50.0
2.88
16.3
33.6
1.40
11.8
26.2
1.08
8.08
112
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
presumed 500 yr median soil organic matter age (Trumbore, 2000;
Six and Jastrow, 2007). The combined tree and soil C, N, and P
sequestration at the catchment scale indicated that the highest
sequestration increments (Mg yr 1) were in the Southern Plains
and Southern Appalachian Mountains, with all the remaining
ecoregions being lower (Table 4).
There was a marked longitudinal trend in N deposition with
highest deposition in the industrial Eastern (Northern and Southern Appalachian Mountains and Coastal Plains) and Midwestern
(Temperate Plains and Upper Midwest) ecoregions and lower in
the Western Mountains and Xeric ecoregions (Table 2). Lower %
sand and a higher DI of catchment soils contribute to higher Catch
DN, and these conditions led to Catch DN values in the Western
Mountains ecoregion significantly higher than in any other ecoregion (Table 3). In-stream DN was highest in the Temperate Plains
ecoregion and lowest in the Northern Appalachian Mountains
ecoregion (Table 3), but when extrapolated to Cum L, Stream DN
was highest in the Southern Appalachian and Western Mountains
ecoregions, and lowest in the Xeric ecoregion. The combined
annual Catch DN and Stream DN were highest in the Temperate
Plains ecoregion and lowest in the Xeric ecoregion (Table 4).
3.2. Economic value of ecosystem services
We used published economic value estimates based on commodity price (water supply), market value (climate regulation), or
damage cost avoidance (water purification) to assess the potential
economic value of ecosystem services provided by headwater
catchments. The economic value of water supply per unit catchment area (INT $ ha 1 yr 1) was higher in the Northern Appalachian and Western Mountains, and lower in the Northern and
Southern Plains, than in the other ecoregions. The weighted
average water supply value for headwater catchments in the
United States was INT $ 245 ha 1 yr 1 (Table 5). Extrapolation of
these values to the entire catchment (INT $ yr 1) resulted in more
similar water supply value among the ecoregions, with the
exception of the Western Mountains ecoregions which was twice
as high as the other ecoregions. The weighted average for headwater catchments was INT $ 470,000 yr 1 (Table 5).
The economic value of climate regulation per unit catchment
area was higher in the Southern Appalachian Mountains and
Coastal Plains ecoregions than in the remaining ecoregions. The
average economic value of climate regulation for headwater
catchments was INT $ 278 ha 1 yr 1 (Table 5). The Northern and
Southern Plains ecoregions, with their larger catchment areas,
sequester more than twice as much C as do the other ecoregions.
Average climate regulation value for headwater catchments in the
United States was INT $ 553,000 yr 1 (Table 5).
We considered three metrics related to the economic value of
water purification, N and P sequestration in catchment trees,
crops, and soil, and catchment and in-stream denitrification. The
value of N sequestration per unit catchment area was greatest in
the Southern Appalachian Mountains ecoregion, followed by the
Coastal Plains and Upper Midwest ecoregions. All other ecoregions
had similarly lower economic value. The average value of N
sequestration was INT $ 7456 ha 1 yr 1. The greatest sequestration of P per unit catchment area was in the Upper Midwest
ecoregion, with lowest sequestration occurring in the Northern
Plains and Xeric ecoregions. The weighted average economic value
of P sequestration by headwater catchments was INT $
2977 ha 1 yr 1. Denitrification per unit catchment area was highest in the Temperate Plains ecoregion and lowest in the Xeric
ecoregion. The average economic value of denitrification by headwater catchments was INT $ 2981 ha 1 yr 1 (Table 5). As was the
case with climate regulation, the larger sizes on the Northern and
Southern Plains catchments increased their potential economic
value as sinks for N and P, or pathways for N removal via
denitrification (Table 5).
Table 5
Mean annual catchment economic value of ecosystem services per unit catchment area (INT $ ha 1 yr 1) and cumulative catchment value ($1000 yr) for water supplya;
climate regulation b, and water purification c provided by headwater catchments of the nine ecoregions. Values ($ ha 1 yr 1) are derived as the product of marginal unit
value and annual ecosystem goods and services from Table 4; those values were multiplied by catchment area to derive the cumulative catchment values ($ yr 1). Bundled
services are the sum of all the measured service catchments for a given ecoregion. Weighted average is the average value, adjusted for unequal sample sizes, of a given
service across the ecoregions, i.e. a national average.
NAPd
Unit Value
3
NPLg
SPLh
TPLi
UMWj
WMTk
XERl
Weighted Average
394
497
292
416
260
420
38
435
41
339
134
410
158
330
350
908
88
216
245
470
$0.12 Mg 1
$ ha 1 yr 1
$1000 yr
241
305
475
677
374
605
112
1,297
156
1,290
123
376
197
412
194
503
153
377
278
553
$160 Mg 1
$ ha 1 yr
$1000 yr
$ ha 1 yr 1
$1000 yr
$ ha 1 yr 1
$1000 yr
$ ha 1 yr 1
$1000 yr
6992
8831
3072
3880
2192
2768
12891
16281
10,384
14,818
2,928
4,178
3,232
4,612
17,311
24,701
9,536
15,401
3,552
5,736
3,152
5,090
16,874
27,252
3,552
40,624
1,712
19,580
2,272
25,985
7,686
87,921
4,672
38,591
2,112
17,445
3,904
32,247
10,885
89,912
4,896
14,952
2,784
8,502
5,088
15,539
13,025
39,779
8,000
16,744
4,608
9,645
2,608
5,459
15,571
32,590
5,376
13,967
2,240
5,820
1,888
4,905
10,048
26,103
4,192
10,337
1,728
4,261
1,293
3,188
7,454
18,379
7,456
15,587
2,977
6,628
2,981
7,544
13,938
30,782
$0.035 m
Climate regulation
C sequestration
Water purification
N sequestration
1
P sequestration
$1600 Mg
Denitrification
$160 Mg 1
a
CPLf
$ ha yr
$1000 yr
Water supply
Total bundled services
by ecoregion
1
SAPe
Krieger (2001), Nunez et al. (2006), Watanabe and Ortega (2011).
Intercontinental Exchange (2012).
c
Keplinger et al. (2003), Sano et al. (2005), US Environmental Protection Agency (USEPA), 2007, Dodds et al. (2009), Turpie et al. (2010), Compton et al. (2011), Watanabe
and Ortega (2011).
d
Northern Appalachian Mountains.
e
Southern Appalachian Mountains.
f
Coastal Plains.
g
Northern Plains.
h
Southern Plains.
i
Temperate Plains.
j
Upper Midwest.
k
Western Mountains.
l
Xeric ecoregions.
b
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
The total potential economic value of these bundled ecosystem
services ranged from INT $7454 ha 1 yr 1 in the Xeric ecoregion
to INT $ 17,311 ha 1 yr 1 in the Southern Appalachian Mountains
ecoregion. The weighted average economic value of these bundled
ecosystem services was INT $ 13,938 ha 1 yr 1 (Table 5). Potential
economic value extrapolated to the whole catchment ranged from
INT $ 16 million yr 1 for catchments in the Northern Appalachian
Mountain ecoregion to INT $ 90 million yr 1 for catchments in the
Southern Plains ecoregion. The weighted average economic value
for headwater catchments in the United States was INT $ 31
million yr 1 per catchment (Table 5).
4. Discussion
Studies of catchment ecosystem services have focused more
often on those services that have established markets, such as
water supply, forest products, and climate regulation (Creedy and
Wurzbacher, 2001; Postel and Thompson, 2005; Nunez et al.,
2006; Brauman et al., 2007; Woodbury et al., 2007; Watanabe
and Ortega, 2011). Fewer studies have considered the interactions
and trade-offs related to multiple ecosystem services (Stenger
et al., 2009; Deal et al., 2012; Watanabe and Ortega, 2011; Kline
and Mazzotta, 2012; Townsend et al., 2012). Our focus is on the
importance of headwater catchments and the ecosystem services
they provide. As such, we limited our considerations to
catchment-scale services that do not limit the future provision of
these ecosystem goods and services. Within this construct, land
cover is a major driver of the kinds and potential amounts of
ecosystem services that can be acquired from headwater catchments, and this construct allows us to compare these services
across a national scale, without the constraint of identified local
markets and users of the services (Wainger and Mazzotta, 2011).
While not the largest or most valuable ecosystem services,
water supply and climate regulation benefit from having established markets for their sale and trading. As such, these services
are among the best studied (Creedy and Wurzbacher, 2001;
Watanabe and Ortega, 2011; de Groot et al., 2012; Townsend
et al., 2012). Our estimates of the economic value of water supply
are similar to climate protection regulation services. However, we
believe our water supply values underestimate the true value of
catchment water dynamics by ignoring the amount of water
necessary to support the maintenance and growth of catchment
biomass, and by not accounting for the climatological effects of ET
(Jansson et al., 1999; Jackson et al., 2005).
Private and public forests in the United States sequester
162 Tg C yr 1, of which trees and soils were the two largest pools
(Woodbury et al., 2007). Catchment C sequestration is related to
forest cover (Creedy and Wurzbacher, 2001; Jackson et al., 2005;
Boix-Fayos et al., 2009). Our estimates of forest C sequestration,
both per unit catchment area and extrapolate to the whole
catchment, are similar to those reported by previous researchers
(Creedy and Wurzbacher, 2001; Jackson et al., 2005; Woodbury
et al., 2007). And, given that headwater catchments represent the
majority of forest lands in the United States, our estimates provide
a unique view of the importance of headwater forest for climate
protection.
Forest C sequestration, largely as trees, has been increasing over
the past several decades (McKinley et al., 2011). Stoichiometric
theory suggests that increases in C sequestration by trees and soil
must be accompanied by increased N and P sequestration (Sterner
and Elser, 2002; Hesson et al., 2004). Sequestration of elements other
than C is infrequently represented in the published literature
(McGroddy et al., 2004; Schade et al., 2005; Cleveland and Liptzin,
2007). Our research uses established C:N:P stoichiometries to extend
the discussion of sequestration to N and P, and the importance of
113
headwater catchments for sequestering N and P with subsequent
economic savings from reduced costs for nutrient removal from
surface waters (Keplinger et al., 2003; Sano et al., 2005; Dodds et al.,
2009; Turpie et al., 2010; Compton et al., 2011; Watanabe and Ortega,
2011). Our results suggest that headwater forests are significant N
and P sinks. From a standing stock perspective, C sequestered in soils
is similar to that stored in trees, but because of their different
stoichiometric ratios, soils sequester significantly more N and P than
do trees. However, on an annual basis trees, because of their younger
median age, sequester C, N, and P at rates that are several times
greater than those for soil. A significant amount of C, N, and P
sequestration is also attributed to agricultural crops (Vitousek et al.,
2009; Post et al., 2012).
Denitrification is a special case of N removal not accounted for
in our sequestration estimates. Seitzinger et al. (2006) reported
that terrestrial catchments accounted for 22% of the global N
removal via denitrification, second only to marine continental
shelf ecosystems. Hofstra and Bouwman (2005), using the same
denitrification model, suggested that agricultural soils may
account more than half of all terrestrial denitrification. By comparison, streams and rivers accounted for 6% of global denitrification. Our estimates for headwater streams suggest even greater
differences between streams and their catchments. From an
N-budget perspective, the sum of Catch DN and Stream DN in
our study catchments exceeds that of N input via wet deposition of
atmospheric N. We report wet deposition because these are
regularly collected as a part of the NADP network, while dry
deposition of N is difficult to measure and is often modeled to
estimate its contribution to the N budget. Because of these
uncertainties, we rely on the empirical NADP wet deposition data,
recognizing that it is likely only half of total (wet þdry) N
deposition (US Environmental Protection Agency (USEPA), 2001;
Anderson and Downing, 2006). Our N accounting also ignores
significant sources of N that could support denitrification, including fertilizer N applied to row crop agriculture and commercial
timber production, and biological N fixation. Together, these N
inputs to catchments average about 40 kg N ha 1 yr 1 across the
conterminous United States (Sobota et al., 2013).
It is widely recognized that ecosystems provide multiple
ecosystem services and there are several studies that demonstrate
the integration of these multiple services (Bennett et al., 2009; de
Groot et al., 2010, 2012; Deal et al., 2012; Kline and Mazzotta,
2012). One of the commonly observed trade-offs related to water
supply is the inverse relationship between catchment water yield
and forest land cover. Forests tend to increase water retention on
the catchment due to a slowing of runoff and increased water lost
through ET (Stednick, 1996; Creedy and Wurzbacher, 2001; Swank
et al., 2001; Brown et al., 2005). Reducing the proportion of the
catchment covered by forests is a one management strategy to
increase runoff for downstream users (Stednick, 1996; Brown et al.,
2005). This increase in water supply is often accompanied by
increased export of base cations, anions, and sediment (Creedy and
Wurzbacher, 2001; Swank et al., 2001). The opposite is true for
afforestation of grasslands and agricultural lands, and the reforestation of harvested timber lands (Jackson et al., 2005; Townsend
et al., 2012). This increased forest development comes with an
additional demand for water and nutrients. Jackson et al. (2005)
referred to this as “trading water for carbon.” Similarly, N and P
sequestered in trees and soils will lead to better water quality of
catchment runoff (Creedy and Wurzbacher, 2001; Swank et al.,
2001; Postel and Thompson, 2005; Deal et al., 2012; Townsend
et al., 2012). Our results demonstrate the complementary and
contradictory relationships among a limited suite of ecosystem
services. These results suggest that management of multiple ecosystem services may not yield win–win scenarios where more than
one ecosystem service is maximized. More likely, management of
114
B.H. Hill et al. / Ecosystem Services 7 (2014) 106–115
multiple ecosystem services will result in an effective balance to
optimize ecosystem services (Bennett et al., 2009; Deal et al., 2012).
We restricted our considerations of ecosystem services to a
limited list of services provided by functioning headwater ecosystems. We would be remiss if we failed to at least mention some of
the other commonly reported ecosystem services. For example, we
considered only the climate regulation and water purification
value of trees, but must also acknowledge the economic value of
trees and forest products, which is reported to be in excess of INT
$250 billion yr 1 for all harvested catchments in the United States
(US Energy Information Administration, 2001). Similarly, we have
not reported on cultural ecosystem services and their economic
value. Krieger (2001) reported that forested ecosystems provide
recreational opportunities that are supported by services and
suppliers for hunting (INT $2 billion yr 1), fishing (INT $3 billion yr 1), and other recreation (INT $6 billion yr 1). He places the
aesthetics and spiritual value of forests in the United States at INT
$1.5 billion yr 1. By comparison, the average economic value of
the bundled ecosystem services we're reporting for our 568
headwater catchments, based on benefit transfer to estimate price
and marginal changes in goods and services produced (water
supply and climate regulation) or damages avoided (water purification), is about INT $ 31 million yr 1 per catchment (INT $
14,000 ha 1 yr 1). Extending these average values to our 568
study catchments yields a cumulative potential economic value
of INT $ 17.5 billion yr 1. The national importance of headwater
catchments, in terms of ecosystem services and their potential
economic value, is even greater when one considers that our study
catchments are a statistical representation of more than 2 million
headwater catchments in the continental United States.
Acknowledgements
The USEPA Office of Water sponsored the National Rivers and
Streams Assessment. AR Olsen supervised the creation of the
survey design. We are indebted to the numerous state, federal,
and contractor field crews and laboratory staff who collected and
analyzed the NRSA samples. The views expressed in this paper are
those of the authors and do not necessarily reflect the views or
policies of the US Environmental Protection Agency, the US Forest
Service, or SRA International. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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